mchess-server/chess/pawn.go

package chess

import (
	"mchess_server/types"

	"github.com/samber/lo"
)

type Pawn struct {
	Color types.ChessColor
}

func (p Pawn) GetAllAttackedSquares(board Board, fromSquare types.Coordinate) []types.Coordinate {
    attackingMoves := make([]types.Coordinate,0,2)
    allMoves := p.GetAllNonBlockedMoves(board, fromSquare)
    for _,move := range allMoves {
       if move.Col != fromSquare.Col {
        attackingMoves = append(attackingMoves, move)
       }
    }
    return attackingMoves
}

func (p Pawn) GetAllNonBlockedMoves(board Board, fromSquare types.Coordinate) []types.Coordinate {
	theoreticalSquares := p.getAllMoves(fromSquare)
	legalSquares := p.filterBlockedSquares(board, fromSquare, theoreticalSquares)

	return legalSquares
}

func (p Pawn) GetColor() types.ChessColor {
	return p.Color
}

func (p *Pawn) HandlePossiblePromotion(b *Board, move types.Move) bool {
	var isPromotionMove bool
	var promotionToPiece types.PieceShortName

	//TODO(m): What if message does not contain a promotion, but should be because a pawn is moved to the end square
	messageContainsPromotion := move.IsPromotionMove()

	if messageContainsPromotion {
		promotionToPiece = *move.PromotionToPiece
	}

	switch move.ColorMoved {
	case types.White:
		if move.StartSquare.Row == types.RangeLastValid-1 &&
			move.EndSquare.Row == types.RangeLastValid {
			isPromotionMove = true
		}

	case types.Black:
		if move.StartSquare.Row == types.RangeFirstValid+1 &&
			move.EndSquare.Row == types.RangeFirstValid {
			isPromotionMove = true
		}
	}

	if isPromotionMove {
		delete(b.position, move.StartSquare)
		b.position[move.EndSquare] = GetPieceForShortName(promotionToPiece, move.ColorMoved)
	}

	return isPromotionMove
}

func (p *Pawn) HandleEnPassant(b *Board, move, lastMove types.Move) bool {
	var wasEnPassant bool

	if lastMove.PieceMoved != types.PawnShortName {
		return false
	}

	switch move.ColorMoved {
	case types.White:
		if lastMove.StartSquare.Row != 7 || lastMove.EndSquare.Row != 5 {
			wasEnPassant = false
			break
		}
		if move.StartSquare.Row != 5 {
			wasEnPassant = false
			break
		}
		if move.EndSquare.Row != 6 {
			wasEnPassant = false
			break
		}
		if move.StartSquare.Col == lastMove.EndSquare.Col+1 &&
			move.EndSquare.Col == lastMove.EndSquare.Col {
			wasEnPassant = true
			break
		}
		if move.StartSquare.Col == lastMove.EndSquare.Col-1 &&
			move.EndSquare.Col == lastMove.EndSquare.Col {
			wasEnPassant = true
			break
		}
	case types.Black:
		if lastMove.StartSquare.Row != 2 || lastMove.EndSquare.Row != 4 {
			wasEnPassant = false
			break
		}
		if move.StartSquare.Row != 4 {
			wasEnPassant = false
			break
		}
		if move.EndSquare.Row != 3 {
			wasEnPassant = false
			break
		}
		if move.StartSquare.Col == lastMove.EndSquare.Col+1 &&
			move.EndSquare.Col == lastMove.EndSquare.Col {
			wasEnPassant = true
			break
		}
		if move.StartSquare.Col == lastMove.EndSquare.Col-1 &&
			move.EndSquare.Col == lastMove.EndSquare.Col {
			wasEnPassant = true
			break
		}
	}

	if wasEnPassant { //play the move
		delete(b.position, lastMove.EndSquare)
		b.position[move.EndSquare] = GetPieceForShortName(move.PieceMoved, move.ColorMoved)
	}

	return wasEnPassant
}
func (p Pawn) getAllMoves(fromSquare types.Coordinate) []types.Coordinate {
	theoreticalMoves := make([]types.Coordinate, 0, 4)

	switch p.Color {
	case types.Black:
		firstMove := fromSquare.Row == types.RangeLastValid-1

		if fromSquare.Down(1) != nil {
			theoreticalMoves = append(theoreticalMoves, *fromSquare.Down(1))
		}
		if firstMove && fromSquare.Down(2) != nil {
			theoreticalMoves = append(theoreticalMoves, *fromSquare.Down(2))
		}

		if lowerRight := fromSquare.Down(1).Right(1); lowerRight != nil {
			theoreticalMoves = append(theoreticalMoves, *lowerRight)
		}
		if lowerLeft := fromSquare.Down(1).Left(1); lowerLeft != nil {
			theoreticalMoves = append(theoreticalMoves, *lowerLeft)
		}

	case types.White:
		firstMove := fromSquare.Row == types.RangeFirstValid+1

		if fromSquare.Up(1) != nil {
			theoreticalMoves = append(theoreticalMoves, *fromSquare.Up(1))
		}
		if firstMove && fromSquare.Up(2) != nil {
			theoreticalMoves = append(theoreticalMoves, *fromSquare.Up(2))
		}

		if upperRight := fromSquare.Up(1).Right(1); upperRight != nil {
			theoreticalMoves = append(theoreticalMoves, *upperRight)
		}
		if upperLeft := fromSquare.Up(1).Left(1); upperLeft != nil {
			theoreticalMoves = append(theoreticalMoves, *upperLeft)
		}
	}

	return theoreticalMoves
}

func (p Pawn) filterBlockedSquares(board Board, fromSquare types.Coordinate, squaresToBeFiltered []types.Coordinate) []types.Coordinate {
	var nonBlockedSquares []types.Coordinate
	//order of movesToBeFiltered is important here
	for _, square := range squaresToBeFiltered {
		pieceAtSquare := board.getPieceAt(square)
		if square.Col == fromSquare.Col { // squares ahead
			if pieceAtSquare == nil {
				nonBlockedSquares = append(nonBlockedSquares, square)
			}
		} else { //squares that pawn attacks
			if pieceAtSquare != nil && pieceAtSquare.GetColor() != p.Color {
				nonBlockedSquares = append(nonBlockedSquares, square)
			}
		}
	}
	return lo.Intersect(nonBlockedSquares, squaresToBeFiltered)
}